KR19990066945A - Liquid Crystal Display Matrix Array Using Obonic Threshold Switching Device to Separate Individual Pixels - Google Patents

Abstract

The invention provides a plurality of liquid crystal display elements (2) distributed in a matrix of rows and columns; Means (10-11) including row and column conductors (4-9), for supplying a video signal and a display element selection signal; And are connected in series between the corresponding row or column conductors 4-9 and the liquid crystal display element 2, respectively, and serve as display element selection devices and current isolation devices, respectively, and have an off-state resistance of 1,000,000,000 Ω or more. It relates to an active matrix liquid crystal display panel comprising a plurality of threshold switching elements (3).

Description

Liquid Crystal Display Matrix Array Using Obonic Threshold Switching Device to Separate Individual Pixels

Related Applications

This application is part of US Patent Application Serial No. 08 / 324,071, filed October 14, 1994 (C.I.P.).

The present invention relates to an image-display panel. More specifically, the present invention relates to a wide-area, high-density, liquid crystal image-display panel for use in television systems and the like. At least as early as cathode ray tubes, much effort has been directed towards the ultimate goal of flat image display panels. Flat-panel display devices have included matrixes of devices such as electroluminescent cells, mechanical shutters, directional particles suspended in a medium, radiation-emitting diodes, gas cells and liquid crystals.

Most prior flat-panel displays have used a matrix of crossed conductors. A light-display element positioned at the intersection of the two conductors with the potential of a given vertical conductor (ie, column conductor) and a given horizontal conductor (ie, row conductor) Activates. In order to ensure partial energization of display elements located anywhere in the row and column conductors, each display element is typically combined with a selection or isolation device that generally takes the form of a series diode or transistor with non-linear characteristics. The selection potential biases the selection device to approximately its characteristic threshold, and the image modulation voltage exerts a usage potential above the threshold of the curve.

In order to solve the problems of these preceding panels, numerous other sources have been proposed. These include the use of commutators, shift registers, traveling wave pulses and similar techniques. However, the results obtained have substantially fallen short of what was required, for example when displaying conventional television images. US Pat. No. 3,765,011 to Sawyer et al. Relates to an image-display panel which has advantages over the preceding panels of the same general nature. In the image-display of the above patent, a pixel has a double-terminal obonic threshold switch or a breakdown-type switch in series with each photo-display element distributed in a panel in a matrix composed of horizontal rows and vertical columns. Separated by coupling. The other columns of the switch were addressed in combination with the pulses and row-select pulses supplied to the switch. At the same time, the other columns were also addressed with corresponding image-display modulation pulses respectively at the instantaneous level of image information. Finally, the other rows were addressed in combination with the column firing the pulses to supplement the voltage needed for firing and with the respective return circuits for the firing and modulation pulses.

Obonic threshold switches in the past were suitable for low-density displays of the past, while these characteristics are not suitable for the high-density displays required today. Thus, the interest of most display manufactures has shifted to other isolation devices. Critical material and device designs of prior art critical switches have proved inadequate to meet the growing demand for wide-area, high-density displays.

Today, the market for wide-area, high-density, liquid crystal displays (LCDs) is $ 4 billion annually and is expected to grow to $ 7 billion in three years. The demand for greater display density and better picture contrast continues to increase. Early displays used liquid crystals in which transparent conductors were compressed between two glass plates. This worthless technology could also be used for low density displays, but using this technology reduces image contrast as the density increases, making the technology unusable for today's displays.

The main challenge facing LCD manufacturers is to have each pixel periodically biased to maintain the color of the pixel. This can be done by applying a voltage to a single addressing row of the display and applying an intensity signal to each addressing column of the display. This will properly bias all the pixels in the row. If there are N rows, each pixel can only be biased for one-Nth of that time. The picture contrast is roughly proportional to this time and decreases very rapidly as the number of rows increases.

The solution is to provide a nonlinear conductor, such as a diode, that acts as an isolation device in the electrical path through each pixel. This pixel can be biased at the potential at which the nonlinear element will conduct. If the pixel is not biased, the nonlinear element will not conduct and eventually the pixel will remain biased. The liquid crystal material has a very low conductivity and this display pixel acts like a 0.2-0.4 pF capacitor. However, since nonlinear conductors are not ideal, there will be charge leakage.

Each pixel in the active matrix LCD display must contain an addressable switching element that serves as a current isolation device. Many techniques have been used for the present application. In evaluating the advantages of the various approaches, several parameters should be considered. The following list includes the nine most important considerations.

1) The current excitation capacity of the switch should be high when the switch is turned on. This allows the pixels to be switched within a short time. The denser the display, the less time it takes for each row pixel to switch.

2) The current leakage of the switch should be low when the switch is turned off. This prevents electrons from leaving a pixel between the times they are addressed. This is very important because of the gray scale potential in the display. This requirement becomes more stringent as the display density increases.

3) The area of the switch must be subtracted from the area of each pixel. This reduces the amount of light that can be projected through the pixels. It is desirable to have a small area switch to increase the aperture ratio of the pixel.

4) If the switch has light sensitivity, a shadow mask is needed to prevent light from reaching the device. A separate process step can be added for this purpose and the aperture ratio can be reduced.

5) Two terminals are required to address an array of pixels in the display. Some switching devices have three terminals, requiring three contacts at each pixel location. This adds complexity to the mask and reduces the aperture ratio.

6) The cost of the display is directly related to the number of process steps in its manufacture. It is desirable to minimize the number of masking steps as possible.

7) Portable computers have been proposed that use low voltages to save battery power. The requirements of the display voltage have limited the making of all low voltage computers.

8) The switching speed of the device should be fast enough that it will not significantly affect the charging of the pixel. As display performance continues to increase, response time will become an increasingly important consideration.

9) The process temperature of the switch should be below the softening point of the substrate. Corning 7059 glass, which is a preferred substrate, has a softening point of 450 ° C. On the other hand, other substrates with higher softening or melting points are much more expensive.

The following table compares nine considerations for a number of prior art display technologies.

An important challenge in today's display technology is to find nonlinear devices with a reasonable ratio of on-current to off-current. Because the display environment is noisy, nonlinear isolation devices must have a pronounced transition from conduction to nonconductive state.

At present, the most common device, an amorphous silicon thin film transistor, has an on-off ratio of about 10 ⁷ . Improving the process may one day increase this ratio to a slightly larger size, but the fundamental drawback of this thin film transistor is that it must be very large to excite the required current in the on state. When the size of the display array reaches 1000 pixels wide by 1000 pixels high, the transistor can no longer supply enough current to adequately bias the pixels. In addition, the remaining considerations face severe limitations in displays that have already been developed.

Polycrystalline thin film transistors provide a solution to the biggest problem (ie, inadequate current in the on-state) of amorphous thin film transistors. Polycrystalline thin film transistors have much larger on-currents than amorphous transistors, but their on-off ratio is worse than the amorphous portion within them. The aperture ratio is improved because polycrystalline thin film transistors require smaller areas to produce. However, this technique requires high temperature manufacturing. This requires expensive substrates and makes the cost of large area displays very expensive. Current displays that have already been developed use the above techniques and can be improved, but it is very complicated to fabricate displays using transistor isolation devices. Polycrystalline transistors add six masking steps to the display manufacturing process. This has a significant impact on yield. Current already developed manufacturing processes are very expensive to produce such displays, but with yields of less than 50%.

Metal-insulator-metal diodes provide a simpler structure than transistor displays and are very easy to manufacture, thus allowing less expensive displays to be produced. Metal-insulator-metal diodes are the simplest means of using non-linear devices in displays and so yields are significantly better. Unfortunately, the ratio of their on current and off current is not very high. The device has a nonlinear characteristic that can provide an on-off ratio up to 5th order. This is sufficient for medium density displays, but modern displays require lower leakage current than this technology can provide. This makes the technique unusable for high density displays, where both high on current and low off current are significant.

Diode isolation is somewhat simpler than transistor isolation, and diodes are as easy as manufacturing MIM devices. Diode isolation provides a good on / off current ratio of about eight orders of magnitude. However, since diodes conduct in only one direction, two pixels are required for each pixel. Thus, each pixel must have two row contacts and one column contact. That's a 50% increase in the number of interconnects to the display, which has limited acceptance of this technology. Thus, diodes provide a useful solution with less masking steps than TFT displays, while the complexity of the cell and the number of contacts are increased.

Prior art threshold switches provide all the desirable properties listed in this paper except for off-state resistance. The threshold switch has the largest on current and fastest response time, enabling a sufficiently large bandwidth for HDTV and the like. This has the widest aperture ratio, enabling better contrast and greater energy efficiency of electro-optics. It is not light sensitive and has a simple device structure, which can be developed into a low cost display. In addition, the device may be operated at lower voltages than any other technology, and therefore would be most desirable for battery operated computers. Only lacking is the very low off current (i.e. high off-state resistance), which allows for accurate gray scale performance.

The present invention relates to threshold switches and chalcogenide threshold switching materials having all the desirable properties of prior art threshold switches and very low off-state currents (high off-state resistance).

Displays with threshold switches take advantage of the simplicity of the MIM display but provide performance that exceeds all other technologies used. Displays for HDTV and future computer displays will require more than 4 million pixels. Current technologies have not been able to meet the demands of such displays before. Attempts at chalcogenide threshold switches using the novel critical switching materials of the present invention will provide high yields because they provide the required performance and are very easy to fabricate.

Summary of the Invention

The present invention relates to an active matrix liquid crystal display panel comprising:

1) a plurality of liquid crystal display elements distributed in a matrix of rows and columns;

2) means for supplying a video signal and a display element selection signal, comprising row and column conductors; And

3) Multiple threshold switching connected in series between the corresponding row or column conductors and the liquid crystal display element respectively, serving as display element selection device and current isolation device, with multiple off-state resistance of 1 × 10 ⁹ Ω Devices and liquid crystal display devices.

The off-state resistance of the preferred threshold switch is about 1 × 10 ¹⁰ Ω. More preferred threshold switch off-state resistance is about 1 × 10 ¹¹ Ω. The off-state resistance of the most preferred threshold switch is about 1 × 10 ¹² Ω.

The critical switching material of the present invention may be prepared from chalcogenide based As-Te, which may additionally comprise one or more components such as Ge, Si, P and Se.

If necessary, a resistance element can be connected in series with the obonic threshold switch in order to properly charge the liquid crystal display element before the current flowing through the threshold switch falls below their holding current.

The present invention relates generally to liquid crystal form information display arrays. In particular, the present invention relates to the use of an obonic threshold switching device using a novel chalcogenide threshold switching material in series with a resistive element as an electrical isolation device in a liquid crystal display array. The obonic threshold switch allows to energize and de-energy selected discrete pixels on the array without affecting other unselected pixels on the array.

1 is a wiring diagram of a liquid crystal Ion (LCD) image-display matrix using an obonic threshold switch as a device cell sector / isolation device;

2 is a wiring representation of a current-voltage diagram for a typical threshold switching device;

3 shows an alternative concept of the invention wherein a resistive element is provided in series with the threshold switch to allow the liquid crystal display element to be adequately charged before the current flowing through the threshold switch falls below their holding current; And

FIG. 4 shows a voltage versus time plot of a typical excitation scheme for a pixel of this display in which a series resistor is coupled with a threshold switch.

The basic matrix 1 for the liquid crystal display panel is shown in FIG.

A plurality of liquid crystal light-display elements 2 are distributed on the panel to clarify the vertical rows connected by the row and column conductors 7, 8 and 9 connected by the row conductors 4, 5 and 6. The individual terminals of each display element 2 are eventually connected between the horizontal and vertical conductors which are almost crossed at the position of the display element. Obonic threshold switch 3 is connected in series with each display element 2 between the row conductor and the column conductor. This obonic threshold switch serves as a current isolation device and a display element selection device.

When a potential exceeding the threshold of an obonic threshold switch is staggered across crossed row and column conductors, the device converts its conduction state high by means well known in the art and by means illustrated in FIG. In the high conduction state, the obonic threshold switch 3 allows current to flow through them, thereby charging the liquid crystal display element 2. This charging current is above the minimum holding current of the obonic threshold switch 3 and keeps the switch conducting. When display element 2 is charged to the required image voltage, the current through threshold switch 3 drops below the holding current and the threshold switch 3 automatically changes to a high resistance state.

Device selection electric pulses for switching the obonic threshold switch 3 to the conduction state and video signal pulses for charging the liquid crystal display device 2 are supplied from a line driver (exciter) and a decoder. The exciter and decoder are shown as "black boxes" 10 and 11 which can be separated into separate units or provided as a display panel mounted on board. The above line drivers and other display exciter circuits required to operate the display are conventional in the art and need not be described in detail here.

In Figure 1, display element 2 acts like a light modulator. In practice, the liquid crystal material is sandwiched between electrodes that are separated by about 5 microns. If no electric field is used, the liquid crystal material polarizes light according to the polarization filter. However, when an electric potential occurs between the electrodes, the polarization angle is changed to disturb the light. Changes in the electric field can change the brightness and give a gray scale.

The technical obonic threshold switch used in the display of U.S. Patent No. 3,765,011 to Sawyer et al., Titled "Transistor Face an Ivisible Foe," on September 19, 1996, at Electronics 191995, page 191-195 The same kind as described in the article by George Cyderis, published. They are described in an article by author H. K. Henish, published September 30, 1969, Scientific American, entitled "Amorphous-Semi-conductor Switching." For example, with regard to displaying conventional television images, Sawyer et al. Used prior art obonic threshold switches that entirely used display matrices such as "similarly small layers or spots of glass-like material placed on the electrodes of corresponding light-display elements." Describe it. The obonic threshold switches used by the inventors are discontinuous thin-film devices produced by metallithography techniques by photolithography, which are much smaller in size and perhaps less than 1 μm in diameter. Each threshold switch exhibits double-stability in that, once applied, current continues to flow through the corresponding display element until the current is disconnected or falls below a threshold called a holding current.

The characteristics of the obonic threshold switch are shown in FIG.

This switch provides high resistance for voltages below the threshold level V _t . When the voltage across the switch is excessive, the switch breaks down and is generally conducted at a constant static voltage V _c ; When conducting, the switch exhibits a small impedance. When the current through the switch drops below the holding current I _h , the switch returns to the high-impedance state, which occurs when the voltage across the switch drops below the lower level V _c . This switching operation is independent of the polarity of the voltage used and switching is fast in both directions.

The reason why prior art obonic threshold switches are not suitable for use as isolation devices in today's high-density, wide-area liquid crystal display panels is because of their off-state resistance. This is obvious because the image element 2 will be discharged through the prior art threshold switch even in the off state. According to sawyer et al., "The off resistance of an ovonic switch may be about 10 ⁷ Ω." This is why Sawyer et al. Used additional capacitors in series with liquid crystal display devices. Because these added capacitors are relatively low off-state resistors, they dissipate their charge through the threshold switch to help the display device remain charged.

The inventors have found all the desired properties of the prior art materials such as high on-state current, low threshold voltage, light-insensitive, simple and low temperature production, high aperture ratio, two terminals and fast switching time and the like. Additionally, new chalcogenide critical switching materials have been fabricated with very high off-state resistance.

In the present invention, the off-state resistance of the obonic threshold switch is about 10 ⁹ Ω or more. The preferred off-state resistance of the critical switch is about 1 × 10 ¹⁰ Ω or more. More preferred off-state resistance is at least about 1 × 10 ¹¹ Ω. The most preferred off-state resistance is at least about 1 × 10 ¹² .

The materials of the present invention are clearly superior to prior art materials in terms of the ability to insulate the display element from other displays and to allow the display element to charge much longer than previously possible.

The critical switching material of the invention is As-Te based on chalcogenide. These additionally include components such as Ge, Si, P, S and Se. Useful compositions include As ₄₁ Te ₃₉ Ge ₅ Si ₁₄ P ₁ , where the subscript is the atomic ratio of each component. This base material can be modified by replacing either or both of As and Te with Se. Examples of these two modified compositions are As ₃₈ Te ₃₇ Ge ₅ Si ₁₄ P ₁ Se ₅ and As ₃₆ Te ₃₄ Ge ₅ Si ₁₄ P ₁ Se ₁₀ . In addition to the modification by substitution of Se, either or both of As and Te may be substituted with additional Si. Examples of Si substitutions are As ₃₈ Te ₃₇ Ge ₅ Si ₁₉ P ₁ and As ₃₆ Te ₃₄ Ge ₅ Si ₂₄ P ₁ .

Further modifications to achieve extremely high off-state resistance may include modifying the coupling structure of the critical switching material in the critical switching device. For example, one may vary the area and thickness of the intersecting portion of the critical material material to increase or decrease the resistance of the critical switch within a certain range.

In a more specific drawing (see FIG. 3), a resistive element 12, such as a resistor, each threshold switch in order to ensure that each liquid crystal display element 2 is adequately charged before the current through the threshold switch 3 falls below its holding current. Can be combined in series with three.

In other words, because each liquid crystal display element (pixel) has a limited capacity and the resistance of the on-state threshold switch is an extremely small pixel, it takes very little time to charge (i.e., this circuit generates very small RC time constants). I have it). Thus, the current flowing through the threshold switch may drop below the holding current before the liquid crystal display element obtains a suitable voltage. Therefore, there are times when it is advantageous to connect a resistor element in series with the threshold switch in order to reduce the flow of current through the threshold switch (i.e., slow down pixel charging) and to properly charge all liquid crystal display elements of a given row time.

For example, the threshold switch has a typical capacitance of about 10 fF and a holding current of about 0.1 microamps, while a typical liquid crystal display device has a capacitance of 0.2 pF. Because of these conditions, a series resistance of about 10 ⁷ Ω is required to properly charge the liquid crystal display device before the threshold switch returns to non-conductive (ie, off) state.

In the absence of a series resistor, the pixel may instantly charge to an inappropriate voltage. A diagram of a general driving scheme for a single pixel will help illustrate this problem. First, a pixel drive voltage is applied to the pixel drive leads (i.e., leads 7, 8 and 9), and then a switching pulse is applied to the threshold switch drive leads (i.e. leads 4, 5 and 6). As the voltage of the switching pulse increases, the potential across the threshold switch increases until the threshold voltage is reached at a time at which the threshold switch is assumed to be in a low resistance state. Almost instantly, the liquid crystal display element is fully charged. However, the voltage charged in the display element is not the correct voltage. This is because the threshold switch returns to the high resistance state before the switching pulse is interrupted. Thus, the display element charges to the added value regardless of what is the pixel drive voltage and the voltage of the switching pulse when the threshold switch is closed. To alleviate this problem, the RC time constant of the circuit must be increased dramatically. This allows the pixel to charge more slowly, thus providing time for the switching pulse to end before the threshold switch is closed. Thus, by including a series resistor, the RC time constant of the circuit can be increased to the point where the pixels are still layered and the threshold switch is in the conduction state even when the switching pulse is over. Thus, the final voltage applied to the pixel will be appropriate.

4 shows an excitation diagram for this display using a series resistor. The uppermost curve represents the switching pulse voltage (alias commonly referred to as the common driver voltage). The curve in the middle stage represents the applied pixel drive voltage (alias is the segment driver voltage). Finally, the lowermost curve represents the pixel voltage (or potential). As can be seen, the use of a series resistor blocks the switching pulse for a long time until the pixel reaches the final, proper voltage.

While certain embodiments of the invention have been shown and illustrated, it is evident that they have been altered and modified without departing from the invention in a broader sense. Accordingly, it is the object of the appended claims to protect all such changes and modifications that fall within the true spirit and scope of the invention.

Claims (21)

A plurality of liquid crystal display elements distributed in a matrix of rows and columns; Means for supplying a video signal and a display element selection signal, comprising row and column conductors; And A plurality of paired thresholds connected in series between the corresponding row or column conductors and the liquid crystal display element, respectively, serving as display element selection devices and current isolation devices, and having off-state resistance of 1 × 10 ⁹ Ω or more; An active matrix liquid crystal display panel comprising a switching element and a resistance element. 2. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element has an off-state resistance of 1x10 < ¹⁰ > 2. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element has an off-state resistance of 1x10 < ¹¹ > 2. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element has an off-state resistance of 1x10 < ¹² > 2. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element comprises a chalcogenide threshold switching element material material disposed between two electrical contacts. 6. The active matrix liquid crystal display panel according to claim 5, wherein the chalcogenide critical switching material is a thin-film body. 6. The active matrix liquid crystal display panel of claim 5, wherein the chalcogenide critical switching material is a arsenic-tellurium based material. 8. The active matrix liquid crystal display panel of claim 7, wherein said chalcogenide critical switching material further comprises one or more elements selected from the group consisting of germanium, silicon, phosphorus, sulfur, and selenium. 9. The active matrix liquid crystal display panel according to claim 8, wherein the chalcogenide critical switching material is As ₄₁ Te ₃₉ Ge ₅ Si ₁₄ P ₁ , wherein the subscript is the atomic ratio of each element. The active matrix liquid crystal display panel of claim 8, wherein the chalcogenide critical switching material is As ₃₈ Te ₃₇ Ge ₅ Si ₁₄ P ₁ Se ₅ , wherein the subscript is an atomic ratio of each element. The active matrix liquid crystal display panel of claim 8, wherein the chalcogenide critical switching material is As ₃₆ Te ₃₄ Ge ₅ Si ₁₄ P ₁ Se ₁₀ , wherein the subscript is an atomic ratio of each element. 9. The active matrix liquid crystal display panel according to claim 8, wherein the chalcogenide critical switching material is As ₃₈ Te ₃₇ Ge ₅ Si ₁₉ P ₁ , wherein the subscript is the atomic ratio of each element. The active matrix liquid crystal display panel of claim 8, wherein the chalcogenide critical switching material is As ₃₆ Te ₃₄ Ge ₅ Si ₂₄ P ₁ , wherein the subscripts are each an atomic ratio of the elements. The active matrix liquid crystal display panel according to claim 1, wherein said critical switching element is disposed on a glass substrate. The active matrix liquid crystal display panel according to claim 1, wherein the threshold switching element has an on-current of about 1 × 10 ⁻³ A or more. The active matrix liquid crystal display panel of claim 1, wherein the threshold switching element has a threshold voltage as low as about 3V. 2. The active matrix liquid crystal display panel of claim 1, wherein the threshold switching element has a switching time of less than about 1000 nsec (nanoseconds). 2. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element has a cross-sectional area for causing an image element to have a switching device aperture ratio of at least about 0.6 or more. The active matrix liquid crystal display panel according to claim 1, wherein said threshold switching element is not sensitive to light. The active matrix liquid crystal display panel of claim 1, wherein the critical switching element is disposed under a temperature of about 300 ° C. or less. 2. The active matrix liquid crystal of claim 1, wherein the critical switching element has a holding current of about 0.1 microamperes, the resistive element has a resistance of about 10 ⁷ ohms, and the liquid crystal display element has a capacitance of about 0.2 pF. Display panel.